The invention relates to the technical field of plasma generation by electron cyclotron resonance (ECR) from a gaseous medium, and more specifically relates to the field of vacuum surface treatments.
As perfectly well known by those skilled in the art, electron cyclotron resonance occurs when an electromagnetic wave and a static magnetic field sufficiently strong for the frequency of the cyclotronic motion of electrons around the magnetic field lines to be equal to the wave frequency are applied at the same time. Thus, electrons can absorb the wave energy to then transmit it to the gas in order to form a plasma. The plasma generated by electron cyclotron resonance may be used in the surface treatment of parts, metallic or not, such as the cleaning of parts by ion etching, the ion assistance to a PVD method, the activation of gas species to form PACVD coatings . . . . Such plasma treatment methods may be used, among others, in mechanics, optics, corrosion protection or surface treatment for the production of energy.
According to the state of the art, many plasma treatments require a source having a plasma with a large extension along an axis. A way to create an extended plasma is to juxtapose several sources of small dimensions. This is for example described in patent EP 1075168 where the plasma is created by juxtaposing several dipolar ECR sources, thus creating a multi-dipolar structure. The configuration described in this patent is illustrated in FIG. 1. Electron cyclotron resonance (ECR) at microwave frequencies is a technology well adapted to such multiple sources since the power distribution can be performed quite easily. It is however difficult, with such a simple juxtaposition of sources, to obtain a very good deposition uniformity. Further, the dipolar structure of such sources does not enable to direct the plasma towards the substrates to be treated, thus generating significant plasma losses towards the walls. Such losses correspond to power losses, which are limiting for the deposition speed.
Other ECR sources have magnetic configurations which decrease losses by directing the plasma more towards the substrates to be treated. This is for example the case for the source described in patent WO 2008/017304 (FIG. 2). As specified in this patent, several of these sources placed side-by-side allow a treatment across a width exceeding the size of a single source. However, such a configuration will not provide a good treatment uniformity, there will necessarily be a drop in the plasma density where the sources touch, due to the magnetic interaction between sources.
There exist other ECR plasma sources having a significant intrinsic extension in one direction. Such sources are described in patents DE 4136297, DE 19812558, and WO 2005/027595. The common point of these sources is that the actual plasma forms a portion of the external conductor of a coaxial structure or of a hollow waveguide. FIG. 3, which corresponds to the prior art of patent DE 19812558, representatively shows the structure of such a source. The source of patent DE 4136297 comprises a hollow cylinder made of a dielectric material which separates the inside of the waveguide from the plasma. The disadvantage of this element is that any conductive deposit on its surface will prevent the forming of the plasma. For example, such a source cannot be used for the stripping of metal parts since the metal vapor due to the stripping would pollute the dielectric. Although the sources of patents DE 19812558 and WO 2005/027595 are potentially less sensitive to such a contamination, they do not optimize the plasma flow towards the parts to be treated either.
The invention aims at overcoming these disadvantages in a simple, sure, efficient, and rational way.
The problem that the present invention aims at solving thus is to provide a linear plasma source having a good uniformity, which directs the plasma towards the substrates to be treated, thus decreasing losses toward the walls, and which may be made insensitive to any conductive deposit forming at its surface.